build: Fixed file format version number in VC12 solution files

Unlike previous versions of Visual Studio the VC12 solution file format
does not increment the format version number, but instead, only changes
the version comment text.

This incorrectly set version number would cause problems for any third
party piece of software that would read the solution file expecting the
version number to be 12.00 and found it to be 13.00, such as some build
accelerators.

Verified against a freshly created solution file which was generated
with VC12.

projects/Windows/VC12/curl.sln

@@ -1,4 +1,4 @@
-Microsoft Visual Studio Solution File, Format Version 13.00
+Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 2013
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "curlsrc", "src\curlsrc.vcxproj", "{5228E9CE-A216-422F-A5E6-58E95E2DD71D}"
 	ProjectSection(ProjectDependencies) = postProject

projects/Windows/VC12/lib/libcurl.sln

@@ -1,4 +1,4 @@
-Microsoft Visual Studio Solution File, Format Version 13.00
+Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 2013
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "libcurl", "libcurl.vcxproj", "{DA6F56B4-06A4-441D-AD70-AC5A7D51FADB}"
 EndProject

projects/Windows/VC12/src/curlsrc.sln

@@ -1,4 +1,4 @@
-Microsoft Visual Studio Solution File, Format Version 13.00
+Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 2013
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "curlsrc", "curlsrc.vcxproj", "{5228E9CE-A216-422F-A5E6-58E95E2DD71D}"
 EndProject